Nowadays, the demand for a solar cell has been increasing in consideration of environment problem and energy problem. The solar cell can be generally classified into “bulk-type” and “thin film-type” according to the configuration. The bulk-type solar cell is made of a wafer obtained by slicing a silicon ingot by a predetermined thickness, and has high conversion efficiency, but the price is high and susceptible to the influence of the material silicon price. On the other hand, there are known thin film-type solar cells such as a “silicon-based thin film solar cell” obtained by forming a thin film of amorphous silicon or polysilicon on a substrate such a glass substrate, a “compound-based thin film solar cell” which is a kind of compound semiconductor and made of copper, indium, selenium, gallium and the like, and an “organic thin film solar cell” to produce photovoltaic power by use of an organic dye. For example, the amount of material silicon used in the thin film-based solar cell is 1/100 or less, and energy required for the manufacturing is less, and thus nowadays the thin film solar cell is drawing attention.
Furthermore, the solar cell is generally classified into a “silicon-based solar cell” and a “compound semiconductor solar cell”, based on the kind of the semiconductor used for the power generation part. Furthermore, the silicon-based solar cell is classified into a crystalline silicon-based solar cell and an amorphous silicon solar cell. The crystalline silicon-based solar cell is classified into a monocrystalline silicon-based solar cell and a polycrystalline silicon-based solar cell.
Regarding conversion efficiency which is the most important aspect for a solar cell, nowadays, the compound semiconductor-based solar cell has reached nearly 25%, which is the highest, and the monocrystalline silicon-based solar cell is about 20%, and the polycrystalline silicon-based solar cell and the amorphous silicon-based solar cell are about 5 to 15%. Regarding the raw material cost, silicon is an element which is the second most abundant on the earth (oxygen is the most abundant), and thus much cheaper than compound semiconductor, and thus silicon-based solar cell is most widely used. The “conversion efficiency” refers to a ratio of the energy converted to electrical energy by the solar cell with respect to the energy of the incoming light into the solar cell. The ratio is represented by “percentage (%)”.
Next, a method of manufacturing a monocrystalline silicon solar cell will be explained briefly. First, a cylindrical silicon single crystal ingot is manufactured by the Czochralski method (the CZ method) or the floating zone melting method (the FZ method) to obtain a silicon wafer which is a substrate of a solar cell. For example, in the CZ method, polycrystalline silicon is supplied into a vitreous silica crucible, followed by melting by heating. Then, a seed crystal is dipped into the obtained silicon melt, and gradually pulled up, to produce a silicon single crystal.
Then, the ingot is sliced to obtain thin wafers each having a thickness of, for example, 300 μm. The surface of the obtained wafer is etched by a chemical to remove processing strain in the surface, to obtain a wafer (substrate) for a solar cell. An impurity (dopant) is diffused into the wafer to form a PN junction on one side of the wafer, and then electrodes are formed on both sides, and thereafter an antireflection film is formed on the sunlight incidence plane to reduce light energy loss due to light reflection, to obtain the solar cell. In the solar cell, in order to increase the current, it is important to manufacture a solar cell having a larger area. The CZ method is excellent in that it allows easy production of a silicon single crystal having a large diameter, and the obtained single crystal is excellent in the strength. Therefore, this method is preferred as a method of obtaining a silicon wafer, having a large diameter, which is a substrate material for manufacturing a large-area solar cell.
On the other hand, in the manufacturing of the polycrystalline silicon-based solar cell, it is preferred to use the casting method where silicon melt is solidified in the mold, or the electromagnetic casting method which is a continuous casting method by electromagnetic induction. By use of such methods, it is possible to manufacture a substrate material at lower cost than a single crystal silicon substrate, which is manufactured by the CZ method. In the casting method, high-purity silicon is heated and melted in a crucible, and a small amount of boron (which is a dopant) and the like is uniformly added thereto, and the silicon melt is solidified therein or solidified after the silicon melt is introduced into a mold. It is required that the crucible and the mold used for the casting method are excellent in heat resistance and shape stability, and in addition, low impurity content is also demanded. So, the crucible is made of silica, and the mold is made of graphite.
The vitreous silica crucible used for manufacturing a silicon crystal is required to have high viscosity at high temperature in order to be durable enough for long-time and multiple pulling or casting. When a silicon single crystal having a large diameter of 300 mm or more is manufactured by the CZ method, a vitreous silica crucible is exposed to a high temperature of about 1500 degrees C. for 300 to 400 hours. It is required that the deformation of the vitreous silica crucible is small even under such conditions. Furthermore, the vitreous silica crucible is disposable, and thus it is demanded to manufacture the vitreous silica crucible easily and at low cost. As a conventional crucible having high strength at high temperature, there is known a crucible having a layer containing aluminium (Al) in high concentration on the outer surface side, a crucible obtained by coating a crystallization promoter (e.g. barium (Ba)) on the outer surface, and a crucible having, on the outer surface, a stabilization layer made of alumina, mullite, or the like (See Patent Documents 1 to 3).